As much I want to believe that Khan got battered by some Brummie piss-head I'm afraid I can't.
What's probably happened is this gang of scallies have seen Khan in this nightclub, started giving him abuse about getting knocked out by Garcia and Khan has probably sparked one of them and caused a big brawl between both sides.
That youtube video called Amir Khan The Truth though is the funniest thing I've seen all year.